This invention relates to a process for the production of epoxide resins from 2,2-bis-(4-hydroxyphenyl)-propane, epichlorohydrin and aqueous alkali hydroxide solution on a continuous basis and also to the purification of the resulting epoxide resin solution and waste water produced.
Processes are known for the production of epoxide resins and in a majority 2,2-bis-(4-hydroxyphenyl)-propane, epichlorohydrin and sodium hydroxide solution are reacted. These processes are accompanied by the formation of the polyglycidyl ethers of 2,2-bis-(4-hydroxphenyl)-propane. The resulting epoxide resins usually consist of mixtures of epichlorohydrin-2,2-bis-(4-hydroxphenyl)-propane-condensation products with differing degrees of condensation.
By altering the 2,2-bis-(4-hydroxphenyl)-propane to epichlorohydrin ratio, the number (n) indicating the chain length of the molecule can be varied. The epoxide resin can thus be set to the desired molecular weight. Epoxide resins are distinguished according to the value of n, as follows:
______________________________________ Epoxide resins of low molecular weight: n = 0 Epoxide resins of medium molecular weight: n = 1 to 2 Epoxide resins of high molecular weight: n = 2 ______________________________________
Methods are known for the non-continuous production of epoxide resins in stirring containers. This process, however, results in considerable fluctuations in the quality of the product, besides which intermittent apparatus only provides a very limited volumetric yield per unit of time.
To improve the efficiency of non-continuous processes, variants have been described in which catalysts or dissolving intermediaries are employed. These variations, however, only result in partial improvements and even exert to some extent unfavorable effect on the quality of the resin, or else they cause additional problems, such as that of separating the dissolving intermediary.
An effect improvement in the intermittent method was obtained by the use of high-speed agitators, such a described in certain patents (U.S. Pat. No. 3,767,618 German Unexamined Specification ecn. No. 2,341,303).
On an industrial scale continuous processes for the production of epoxide resins operate far more economically than non-continuous methods.
A description has been given of a process for the continuous production of epoxide resins of low molecular weight, in which the synthesis of the resin is carried out in a number of stages in a stirring cascade (U.S. Pat. No. 2,840,541), a mixture of epichlorohydrin and 2,2-bis-(4-hydroxyphenyl)-propane being caused to react with soda lye in two to three reaction zones. When 2 reaction zones are adopted, 40-75% and preferably 65% of the soda lye employed is fed into the 1st zone and the remainder into the 2nd zone. The throughput in the 1st zone amounts to 1 mol dian 2,2-bis-(4-hydroxphenyl)-propane per hr and liter space of the reaction chamber. The resin obtained as a molecular weight of about 380-420.
Systems are also known in which, by the use of dissolving intermediaries, the time for which the mixture has to remain in the reaction vessels can be shortened (U.S. Pat. No. 2,848,439). The dissolving intermediary used in this case consists of an aliphatic secondary alcohol. The reaction temperature ranges from 70.degree. to 80.degree. C. The duration amounts to 2 hrs and the throughput to 0.23 mol dian per hr. and liter reaction chamber.
In the known processes, however, the reaction times are still very long, so that only slight volumetric yields per unit of time are obtainable. In the case of cascade type stirring apparatus, furthermore, mixtures occur in each reaction chamber which have an unfavorable effect on the quality of the resin.
In order to mitigate these drawbacks, use is made of cascade typestirring apparatus with a large number of reaction vessels. According to U.S. Pat. No. 2,986,551, for instance, the operation is carried out in a cascade-type stirring apparatus with 6 reactors. The dissolving intermediary employed consists of acetone. The alkali lye is fed in stagewise, in quantities of 12.5 to 45% in each case. The duration amounts to 3-15 min. in each stage, with temperatures of 100.degree.-180.degree. C. The resin produced has a molecular weight of 366 and a viscosity of 150 P at 25.degree. C.
According to U.S. Pat. No. 3,069,434 likewise, a cascade with 6 reactors is used. The dissolving intermediary used consists of alcohol. The alkali lye is fed into each stage. The temperature amounts to 50.degree.-80.degree. C. and the duration is 10-20 min. per stage.
The large number of reactors and the resulting gradual conversion of the reagents certain reduces the mixture effects, but very large production plant is required.
Continuous processes for the production of epoxide resins, dispensing with reaction vessels, offer technical advantages over the foregoing.
All continuous processes hitherto known operate either with auxiliary materials such as dissolving intermediaries or amines, thus suffering from serious drawbacks, or with long reaction times, providing only very limited volumetric yields per unit of time.
The use of dissolving intermediaries increases the cost of the process, renders it more difficult to work up the reaction mixture and usually entails expensive separating operations. Increased expenditure on safety systems is likewise involved. In the catalytic processes, not only the cost of the catalysts has to be considered (frequent use being made of amines of quaternary ammonium salts) but difficulties are also encountered in the separation of the catalysts, so that the quality of the resins is frequently unsatisfactory.
Furthermore, the relatively low throughput speeds necessitate disproportionate dimensions for the production plant or the use of a large number of reactors.
It is also generally known that the production of epoxide resins is accompanied by that of crude resins contaminated with inorganic compounds, such as NaCl and NaOH, as well as organic compounds, such as glycerine, polyglycerine and undefined crosslinked polymers. These contamination agents enter the waste water after the purification and thus cause environmental pollution.
A decisive factor for the quality of the resin is that these impurities should be removed from the latter, as far as possible in their entirety. As regards the waste water, it is essential that the residual resins and the impurities should likewise be totally removed.
The purification method adopted is generally one in which the crude resin is given an addition of an organic solvent, such as an aromatic substance, a cycloaliphatic, a lower alcohol or a low ketone, after which the resulting suspension, consisting of the resin solution and the solids, is subjected to a purification process.
Systems are known in which the solids of such suspensions are removed by filtering or centrifuging processes (USSR Pat. No. 191,118, Swiss Pat. No. 382,448, German Examined Specification No. 1,593,819). In such cases use is made of a solvent in which the inorganic compounds are insoluble. For the complete separation of the inorganic salts the resin solution is dehydrated and then filtered or centrifuged.
This purification method has been found inadequate, as the resin solution then still retains glycerine and polyglycerine, which adversely affect the quality of the resins, particularly their transparency.
Systems are also known in which the resin suspension is purified by washing it with water and decanting it from the solid polymers (U.S. Pat. No. 2,839,494, German Unexamined Specification No. 2,217,239).
Owing to the unsatisfactory phase separation the above washing process is usually performed intermittently in a stirring unit.
In this method the resin is absorbed in an organic solvent, such as toluene. The inorganic compounds and the glycerines are then dissolved out with water. Owing to the density difference the organic phase separates from the aqueous phase. Difficulties in the phase separation occur if any appreciable quantities of interfacial active substances, such as polyglycerines, are formed during the condensation. In this case the phase separation time amounts to a few hours. To enable the phase separation to be carried out more rapidly, therefore, it is recommended that the resin solution be neutralized (U.S. Pat. No. 2.824,855, German Unexamined Specification No. 2,106,788, U.S. Pat. No. 3,309,384, U.S. Pat. No. 2,879,259, U.S. Pat. No. 2,848,435) or that the washing water be given an addition of an inorganic compound by which the phase separation is accelerated (German Examined Specification No. 1,116,398). To enable the inorganic compounds to be washed out in their entirety, the washing process must be carried out 4-5 times, about 500 ml of water being required for each liter of resin solution. Owing to the long settling times involved the volumetric yields per unit of time are very limited.
It is also known that the washing operation can be performed continuously in cascade-type stirring units with separating containers.
The known processes suffer from the following drawbacks:
As in the case of filtering or centrifuging, the resin still retains glycerine and polyglycerine. PA1 As when the resin is washed in the stirring unit, the washing operation is non-continuous, involving considerable expenditure of time and large quantities of washing water. PA1 As the case of the washing operation in the cascade-type agitating apparatus, a large amount of apparatus is required and a great deal of space occupied. PA1 The waste water containing the resin residues has to be discarded, thus causing environmental pollution.